diff --git a/law/contrib/git/__init__.py b/law/contrib/git/__init__.py
index 76367fb2..539095ea 100644
--- a/law/contrib/git/__init__.py
+++ b/law/contrib/git/__init__.py
@@ -8,6 +8,7 @@
 
 
 import os
+import threading
 import subprocess
 from abc import abstractmethod
 
@@ -43,6 +44,7 @@ def __init__(self, *args, **kwargs):
         super(BundleGitRepository, self).__init__(*args, **kwargs)
 
         self._checksum = None
+        self._checksum_lock = threading.RLock()
 
     @abstractmethod
     def get_repo_path(self):
@@ -53,16 +55,23 @@ def checksum(self):
         if self.custom_checksum != NO_STR:
             return self.custom_checksum
 
-        if self._checksum is None:
-            cmd = [rel_path(__file__, "scripts", "repository_checksum.sh"), self.get_repo_path()]
-            cmd = quote_cmd(cmd)
-
-            code, out, _ = interruptable_popen(cmd, shell=True, executable="/bin/bash",
-                stdout=subprocess.PIPE)
-            if code != 0:
-                raise Exception("repository checksum calculation failed")
-
-            self._checksum = out.strip()
+        with self._checksum_lock:
+            if self._checksum is None:
+                cmd = quote_cmd([
+                    rel_path(__file__, "scripts", "repository_checksum.sh"),
+                    self.get_repo_path(),
+                ])
+
+                code, out, _ = interruptable_popen(
+                    cmd,
+                    shell=True,
+                    executable="/bin/bash",
+                    stdout=subprocess.PIPE,
+                )
+                if code != 0:
+                    raise Exception("repository checksum calculation failed")
+
+                self._checksum = out.strip()
 
         return self._checksum
 
diff --git a/law/contrib/mercurial/__init__.py b/law/contrib/mercurial/__init__.py
index ca18e6ea..2baa1455 100644
--- a/law/contrib/mercurial/__init__.py
+++ b/law/contrib/mercurial/__init__.py
@@ -8,6 +8,7 @@
 
 
 import os
+import threading
 import subprocess
 from abc import abstractmethod
 
@@ -18,7 +19,7 @@
 from law.target.local import LocalFileTarget
 from law.parameter import NO_STR, CSVParameter
 from law.decorator import log
-from law.util import rel_path, interruptable_popen
+from law.util import rel_path, interruptable_popen, quote_cmd
 
 
 class BundleMercurialRepository(Task):
@@ -43,6 +44,7 @@ def __init__(self, *args, **kwargs):
         super(BundleMercurialRepository, self).__init__(*args, **kwargs)
 
         self._checksum = None
+        self._checksum_lock = threading.RLock(())
 
     @abstractmethod
     def get_repo_path(self):
@@ -53,15 +55,23 @@ def checksum(self):
         if self.custom_checksum != NO_STR:
             return self.custom_checksum
 
-        if self._checksum is None:
-            checksum_script = rel_path(__file__, "scripts", "repository_checksum.sh")
-            cmd = [checksum_script, self.get_repo_path()]
-
-            code, out, _ = interruptable_popen(cmd, stdout=subprocess.PIPE)
-            if code != 0:
-                raise Exception("repository checksum calculation failed")
-
-            self._checksum = out.strip()
+        with self._checksum_lock:
+            if self._checksum is None:
+                cmd = quote_cmd([
+                    rel_path(__file__, "scripts", "repository_checksum.sh"),
+                    self.get_repo_path(),
+                ])
+
+                code, out, _ = interruptable_popen(
+                    cmd,
+                    shell=True,
+                    executable="/bin/bash",
+                    stdout=subprocess.PIPE,
+                )
+                if code != 0:
+                    raise Exception("repository checksum calculation failed")
+
+                self._checksum = out.strip()
 
         return self._checksum